In an x-ray computed tomography apparatus, a gantry apparatus is provided with a light projector therein for positioning of a patient. A visible light beam emitted from the light projector passes through a transmission film provided in the housing of the gantry apparatus and is incident on the patient and the bed. In general, such a transmission film is formed as a transparent or translucent film, and therefore the inside of the gantry housing may be visible through the film to the patient who is lying on the bed. If the inside of the gantry housing is visible during x-ray CT imaging, the eyes of the patient tend to follow a rotation unit that is rotating inside the gantry housing at high speed. This may cause the head of the patient to move, which may result in body motion artifacts.